a) Field of the Invention
The present invention relates to an acoustic lens system to be used for ultrasonic imaging in an ultrasonic system which displays an ultrasonic image of an object while transmitting ultrasonic waves and receiving the ultrasonic waves reflected by the object.
b) Description of the Prior Art
An ultrasonic system of this type comprises, as shown in FIG. 1, a transducer 1 which is composed of an array of minute ultrasonic elements arranged in a pattern of lattice, and an acoustic lens system 2 which is made of polystyrene or a similar material and located between the transducer and the object. Each of the ultrasonic elements is so adapted as to transmit ultrasonic waves under excitation by a pulse generator 3 and receive the ultrasonic waves reflected by the object (the ultrasonic element serves as a transmitter and also as a receiver). The spaces reserved between the transducer 1 and the acoustic lens system 2 and between the acoustic lens system 2 and the object are filled with water or the similar substance.
First, one of the ultrasonic elements transmits ultrasonic pulses, which are focused on the object by the acoustic lens system 2. The ultrasonic pulses reflected by the object are focused reversely on an original ultrasonic element by the acoustic lens system 2 and transduced into electrical signals by the ultrasonic element. Then, the neighboring ultrasonic element functions in a similar manner. Upon completing the scanning of one line after the repetition of the similar manner, the scanning proceeds to the next line. By operating all the ultrasonic elements as described above, an entire range covering the object is scanned by the ultrasonic waves. The electrical signals thus obtained are processed by a signal processing circuit 4 for displaying an ultrasonic image of the object on a monitor TV 5.
The conventional acoustic lens systems of the type described above have already been disclosed, for example, by Japanese Patent Preliminary Publication No. Sho 51-113601 and U.S. Pat. No. 3,979,711.
However, in these documents, as the acoustic lens systems of the above-described type, biconcave single lenses made of substances having the velocity of sound of the inside higher than that of water are merely illustrated and detailed analyses are not made as to conditions required to secure the acoustic lens systems suitable for the above system. As a result, clarification is not made as to how the acoustic lens systems which have wide field angles and good resolution, can be acquired and neither are the acoustic lens systems having imaging performance sufficient for practical use realized. Further, the conventional acoustic lens systems adopt no antireflection films and therefore allow reflection to be caused due to difference in acoustic impedance, thereby posing a problem that ultrasonic waves are remarkably attenuated due to lowering of transmittance therefor. In addition, the conventional acoustic lens systems have another problem that the lens systems allow spurious images, namely ghost, to be formed due to multiple reflections on the lens surfaces.